I'd add that in my opinion, focusing on mental illness as a response to gun control supporters is mostly a way for gun folks to lay blame on some "other" group... "Oh, no, it's not us nice gun owners who are the problem, it's all those crazy people out there."

It's seriously wrong from a civil rights point of view, and when it's applied to anti-gun folks, it's a strategy with potential to backfire: once you introduce "hoplophobia" (silly term) into the discussion, why shouldn't the other side argue for a diagnostic category called "hoplophilia"? [sarcasm] "Surely anyone who thinks he needs that many guns must be mentally ill..."[/sarcasm]